This relates to plating processes, and more particularly to means for supporting a printed circuit board immersed in a liquid plating solution between plating manifolds, for selective repositioning with respect to the manifolds to enhance plating uniformity.
Deposition of metallic layers upon printed circuit boards is well known and can be accomplished by electroplating or electroless plating techniques. Electroplating typically is accomplished in a tank containing an electrolyte rich in ions of a metal to be plated. An object to the plated is immersed in the electrolyte solution and electrically coupled as a cathode. An object of the plating metal also is immersed in the electrolyte and connected as an anode, to provide the ions. Electroless plating is accomplished by a chemical reduction of a metallic ion in an aqueous solution and subsequent deposition of the metal, without applying electricity.
Both electroplating and electroless plating processes are most readily employed with objects having relatively flat and smooth surfaces. Special approaches have been attempted to counter problems encountered in plating rough or irregular objects. For example, U.S. Pat. No. 4,262,044 (Kuczma) discloses an apparatus for the electroless nickel plating of particularly long bodies such as piping or tubing. The object to be plated is disposed vertically in a tall tank containing an electroless nickel solution. Within the tank is a fluid distribution system including a pump, filter, heat exchanger to heat the solution, and a sparger tube with outlets directed toward the body being plated. The distribution system is said to maintain a uniform temperature, concentration and pH of the solution throughout the tube, all for a more uniform plating.
Another approach to electroless plating is disclosed in U.S. Pat. No. 3,895,137 (Avramidis). An electroless nickel plating solution is heated to incipient boiling in order to agitate the solution. Further, air is evacuated from the plating tank in which a chain or similar article is immersed. This approach is designed to improve electroless plating action, particularly along closely spaced surfaces of the chain links.
One difficulty, encountered frequently in connection with the plating of printed circuit boards, arises in connection with depositing metal in through-holes. As the circuit boards have become more sophisticated, through-holes have become small, for example twenty thousandths of an inch in diameter, with a length equal to the printed circuit board thickness, for example 0.3 inches. A severe difficulty is presented in gaining access to interior walls forming the through-holes, whether with an electrolyte or an electroless solution. An added problem, encountered particularly in electroplating, is the nature of the electrical field surrounding a through-hole, i.e. strongest near the mouth or entry of the through-hole and diminishing along its length. The result is a correspondingly low plating rate along interior walls as compared to plating at the mouth of the hole and other areas of the circuit board being plated.
To cover through-holes by electroless plating, U.S. Pat. No. 4,622,917 (Schramm) discloses a tank in which a pair of inwardly facing parallel manifolds are immersed on opposite sides of a printed circuit board. The electroless plating solution is pumped through a nozzle matrix of one of the manifolds, and is simultaneously drawn away from the printed circuit board through a second matrix of nozzles in the opposite manifold. This creates a pressure differential across the circuit board, enhancing the overall plating rate and increasing uniformity of plating over the walls defining through-holes.
In connection with electroplating, U.S. Pat. No. 4,174,26l (Pellegrino) discloses supporting a plated article above the electrolyte surface as a cathode between two anode manifolds, each with an array of spray nozzles. A second array of suction openings is intermixed with each array of spray nozzles. Moreover, the opposing manifolds can be offset so that spray nozzles of one of them are aligned such that their centers of delivery are offset with respect to each other. This improves the plating of through-holes in printed circuit boards, as it allows the electrolyte to be forced through the holes due to the localized pressure differential in the area of each through-hole.
The plating of circuit boards using manifolds having multiple nozzle arrays has thus been found beneficial, particularly when pressure nozzles are interspersed with arrays of suction openings. This geometry, critical for through-hole plating, becomes a shortcoming in surface plating distribution. Along with improved plating rates and plating of through-holes, a continuing problem of such arrangements has been a lack of uniform thickness on the surface and in holes, over the plated layer. Agitation of the plating solution and oscillation of the printed circuit board or other object being plated have been attempted, yet these solutions are overly complex or otherwise unsatisfactory.
Therefore, it is an object of the present invention to improve plating uniformity in a system employing manifolds with arrays of pressure nozzles or orifices for directing plating solution toward a printed circuit board.
Another object is to overcome a plating rate differential, arising in multiple nozzle systems, between surface areas proximate the nozzles and voids between nozzles, without oscillating the object being plated.
Yet another object is to provide a reliable, low-cost means for selectively supporting a printed circuit board in a plurality of alternative positions with respect to multiple nozzle plating manifolds on opposite sides of the printed circuit board.